Quasars as standard candles

IV. Analysis of the X-ray and UV indicators of the disc-corona relation

¹ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
e-mail: matilde.signorini@unifi.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
³ University of California-Los Angeles, Department of Physics and Astronomy, PAB, 430 Portola Plaza, Box 951547 Los Angeles, CA 90095-1547, USA
⁴ Scuola Superiore Meridionale, Largo S. Marcellino 10, 80138 Napoli, Italy
⁵ Istituto Nazionale di Fisica Nucleare (INFN), Sez. di Napoli, Complesso Univ. Monte S. Angelo, Via Cinthia 9, 80126 Napoli, Italy
⁶ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA

Received: 8 February 2023
Accepted: 19 June 2023

Abstract

Context. A non-linear relation between quasar monochromatic luminosities at 2500 Å and 2 keV holds at all observed redshifts and luminosities, and it has been used to derive quasar distances and to build a Hubble diagram of quasars. The choice of the X-ray and UV indicators has so far been somewhat arbitrary and has typically relied on photometric data.

Aims. We aim to determine the X-ray and UV proxies that provide the smallest dispersion of the relation in order to obtain more precise distance estimates and to confirm the reliability of the X-ray-to-UV relation as a distance indicator.

Methods. We performed a complete UV spectroscopic analysis of a sample of ∼1800 quasars with SDSS optical spectra and XMM-Newton X-ray serendipitous observations. In the X-rays, we analysed the spectra of all the sample objects at redshift z > 1.9, while we relied on photometric measurements at lower redshifts. As done in previous studies, we analysed the relation in small redshift bins, using fluxes instead of luminosities.

Results. We show that the monochromatic fluxes at 1 keV and 2500 Å are, respectively, the best X-ray and UV continuum indicators among those that are typically available. We also find a tight relation between soft X-ray and Mg IIλ2800 Å line fluxes, and a marginal dependence of the X-ray-to-UV relation on the width of the Mg II line.

Conclusions. Our analysis suggests that the physical quantities that are more tightly linked to one another are the soft X-ray flux at ∼1 keV and the ionising UV flux blueward of the Lyman limit. However, the ‘usual’ monochromatic fluxes at 2 keV and 2500 Å estimated from photometric data provide an almost as tight X-ray-to-UV relation, and can be used to derive quasar distances. The Hubble diagram obtained using spectroscopic indicators is fully consistent with the one presented in previous papers, based on photometric data.